A double-base solid propellant is a solid propellant composition including a cellulose nitrate (a nitro-cellulose, NC) and at least one nitric ester such as nitroglycerine (NGl), trimethylolethane trinitrate (TMETN), triethylene glycol dinitrate (TEGDN) or butanetriol trinitrate (BTTN). According to the applications, the solid propellant also includes a plasticizer and other additives such as ballistic property modifiers, stabilizers and antiinstabilities.
These propellants are manufactured by following two types of processes which are well known to a person skilled in the art: a first type of process known as solventless (EDB) and a second type of process known as casting moulding.
A plastic-bonded double-base solid propellant is a solid propellant composition including an energetic binder based on at least one nitric ester and optionally a crosslinkable prepolymer, in which are incorporated oxidising or reducing fillers, a nitramine such as RDX (hexogen) or HMX (octogen) and additives such as those mentioned above.
These propellants are manufactured by following two types of processes which are also well known to a person skilled in the art: a first, so-called casting moulding, process derived from that mentioned above in the case of double-base propellants and a second type of process, known as an overall method or slurry casting method.
Details concerning these processes will be found in the work by A. Davenas "Technologie des propergols solides" Masson 1989, or "Solid rocket propulsion technology" Pergamon Press 1993.
The double-base or plastic-bonded double-base solid propellants often require the use of ballistic property modifiers or of compositions which modify the ballistic properties, especially their rate of burning. In fact, the rate of burning of a propellant is generally an increasing function of the pressure. To improve the operating conditions of an engine containing solid propellant, it is desirable that this rate of burning should remain substantially constant within a given pressure range, which is known as the plateau effect, or else that the rate of burning should decrease in this pressure range, which is known as the mesa effect. These modifications of the rate of burning of the solid propellant are obtained with modifiers or compositions which modify the ballistic properties.
The mechanisms of action of these modifiers of burning have not yet been fully clarified. Various organometallic salts and various oxides are known to modify the ballistic properties of double-base or plastic-bonded double-base solid propellants, especially organometallic lead salts and lead oxides, which are widely known to be effective.
However, the toxicity of lead involves special precautions in the storage and the use of these ballistic property modifiers during the manufacture and the use of the propellant. Finally, the presence of these lead-based modifiers is detrimental to the environment when the propellant is burning.
Manufacturers of propellants have investigated other, nontoxic, compounds in order to replace these lead salts or oxides in their function as a modifier of ballistic properties. Solutions appear to be possible when copper or barium compounds are employed. However, while the function of a modifier of the ballistic properties of the solid propellant appears to be capable of being fulfilled in an equivalent manner and of solving the toxicity problem, other properties or performance characteristics of the propellant are impaired by the use of these copper or barium compounds. Solid propellants containing solely copper salts as a ballistic property modifier generally exhibit a poor fitness for cracking aging, and this considerably reduces the size of the blocks of solid propellant which can be manufactured with such compositions (reference will be made to the work by Davenas, already referred to, for comments on cracking aging). Barium salts, being highly soluble in water, do not lend themselves well to some stages of the manufacture of the solid propellants according to the abovementioned processes.